Sigmodontines are a New World group; currently they range from Tierra del Fuego, the southernmost part of South America, north to the southern United States. They are also distributed on some islands adjacent to the continent, as well as in some oceanic islands as the Galápagos archipelago. However, most of extant sigmodontine diversity is confined to the South American continent. In fact, 61 living genera are endemic to South America and the neighboring islands such as Trinidad. Three genera: Oligoryzomys, Oryzomys, and Sig-

The saltmarsh harvest mouse (Reithrodontomys raviventris) is active primarily at night, but may be seen during the day. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)
Brush mice (Peromyscus boylii) are omnivorous. (Photo by Anthony Mer-cieca/Photo Researchers. Reproduced by permission.)

modon inhabit areas of the three Americas. Seven genera are distributed in Central and South America, although three of these, Ichthyomys, Oecomys, and Rhipidomys, have only Panama as their extra South American part of the range. The water mouse Rheomys is the only mainland genus not present in South America. In addition, the living genus Nesoryzomys is endemic to the Galápagos Islands. Finally, three genera that presumably became extinct in historic time: Megalomys, Megaoryzomys, and Noronhomys were endemic, respectively, to the Lesser Antillas, and the oceanic Galápagos and Fernando de Noronha Islands. However, this last statement may change since an undescribed fossil from mainland Argentina may be assignable to Noronhomys (Pardiñas, pers. com.).

The past distribution of the subfamily as a whole roughly matches current distribution. Lack of fossil records from some areas (e.g., most of the Amazon basin or the highland Puna) is probably due to the lack of adequate sediment beds and/or insufficient exploratory work. However, it is worth noting that the distributions of several genera, such as Bibimys, Kunsia, and Pseudoryzomys, have markedly shifted in a few thousand or even hundred years (Pardiñas, 1999).

It has to be emphasized that much still remains to be learned about sigmodontine distribution, as is shown by the results of field work conducted in the late twentieth and early twenty-first centuries, which prompted extensions of the sig-modontine known distribution. For example, the genus Rhagomys, previously known from the Atlantic coastal Brazilian state of Rio de Janeiro, has been reported from the eastern slopes of the Peruvian Andes (Luna and Patterson, 2003). A less spectacular example, although still remarkable was reported by Emmons in 1999, who collected in eastern Bolivia specimens of Juscelinomys, a genus previously considered to be restricted to the Brazilian Federal District.

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